Liquid crystal devices and materials have been finding new applications, such as electro-optically active windows, sunroofs and information displays. Thin films containing microdroplets of liquid crystal material dispersed within polymers are capable of switching between a light scattering or opaque state and a clear transparent state when responding to applied electric fields. Traditional applications for this technology include electro-optical devices, information displays and other electronic devices. In the fabrication of these information displays and electronic devices, the visible light wavelengths were traditionally the wavelengths of concern.
For other applications, including windows and sunroofs in automotive applications, it would be especially advantageous to have a liquid crystal film which would not only scatter visible light to act as a light shutter, but one which would also scatter near-infrared solar radiation so the film may also act as a solar heat load attenuator. It would also be advantageous to fabricate architectural windows including a film capable of not only scattering light, but also of scattering infrared solar radiation.
Still in other applications, including projection systems of visual information, it may be desirable to reduce excessive light intensity at light wavelengths longer than about 555 nanometers, which is the wavelength at which the human eye is typically most sensitive, because light with wavelengths longer than 555 nanometers may have undesirable heating effects or it may wash out the crispness of the image being projected.
In the past, it has been shown that liquid crystal materials can be mixed with certain liquid polymer precursors, cast into a film, and phase separated by curing to form microdroplets of liquid crystal in situ from a homogeneous solution of the liquid crystal and the polymer. The polymer material which is formed by curing functions as a support medium for the microdroplets of liquid crystal material contained therein.
The microdroplets of liquid crystal material dispersed in the polymeric supporting matrix act as light scattering particles which, when in the "off" state, scatter light, making the film substantially opaque. When the film is in its "on" state, it is transparent and allows light through the film. This ability to act as a light shutter lends the liquid crystal material to many applications for giving selective darkness or opaqueness to glass or plastic enclosures, whether its application is automotive, commercial or electronic. Depending upon the nature of the liquid crystal material and the temperature or voltage of the film, the film may be opaque or transparent at room temperature. When the film has a voltage applied or is heated, the film may change from opaque to transparent or from transparent to opaque depending upon the construction of the device.
It has been determined previously that in order to optimize the light scattering qualities, the films should contain a maximum amount of liquid crystal droplets. In addition, the amount of liquid crystal within the droplets should also be maximized as opposed to remaining dissolved in the matrix. Previous attempts have been made to maximize the amount of liquid crystalline material entrapped in the droplets. The liquid crystalline films were applied onto an electrode-coated substrate. The electrode coatings may be indium-tin-oxide or fluorine-tin-oxide or other transparent conducting electrode materials for applying the required electrical voltage. In the final product, the liquid crystal film is sandwiched between two substrates coated with the electrode material, with the electrodes in electrical communication with the liquid crystal film.
Prior art patents have disclosed means and methods for first dissolving liquid crystals into an uncured polymer, and then curing the mixture to cause the liquid crystals to phase separate into microdroplets for scattering of light. A single phase solution is a miscible mixture of liquid crystal and polymer. Upon phase separation, the materials separate into two discrete phases, i.e., a liquid-crystal-rich phase, and a polymer-rich phase. Phase separation has been achieved in the past by (1) cooling a resin heated above the transition temperature, thereby causing dissolution and precipitation of the liquid crystals, (2) curing the unpolymerized or uncured resin containing liquid crystals, and (3) evaporating solvents from a solvent-polymer-liquid crystal system. A particular method may be selected for an individual material due to its distinctive physical characteristics.
Previously described and disclosed materials for liquid crystal films included a single droplet size distribution peaked about a single characteristic value. Several prior art patents, including U.S. Pat. No. 4,673,255 issued to West et al, U.S. Pat. No. 4,685,771 issued to West et al, and U.S. Pat. No. 4,435,047 issued to Fergason, disclose materials and methods for making the same which include uniformly sized droplets. The methods for achieving uniformly sized droplets have included sorting, sieving and other methods.
A polymer dispersed liquid crystal system is said to be polydisperse if it possesses a multimodal or sufficiently broad distribution of microdroplet sizes as defined in terms of either radius or volume. A multimodal distribution is a distribution which is bimodal, trimodal or of a greater order modality. Thus, a polymer dispersed liquid crystal film simultaneously containing microdroplets with local maxima centered at one micrometer and five micrometers is said to have bimodal polydispersity with mean diameters of one and five micrometers respectively. Likewise, a polymer-dispersed liquid crystal film simultaneously containing microdroplets with local maxima centered at one, three and ten micrometers is said to have trimodal polydispersity with mean diameters of one, three and ten micrometers respectively. In general, the modes of a population are the values of the variate for which the relative frequency attains a local maximum. Prior art polymer-dispersed liquid crystal materials have had only one such maximum. A film having such properties would be said to be unimodal.
Accordingly, it is an object of the present invention to provide a polydisperse liquid crystal material including a multimodal distribution of liquid crystalline microdroplets having at least two local maxima in the distribution of droplet diameters.
In addition, a very high order modality distribution of droplet sizes may appear to be a broad distribution if the mean diameters of the individual modes of the distribution are very numerous and closely spaced relative to each other. In such a distribution it may be difficult to distinguish between the individual peaks or maxima characterizing the various modes, but the distribution may still function as an essentially polydisperse distribution provided that the width of the distribution is sufficiently large. Such a broad distribution may be conveniently characterized by a spread parameter which, if larger than about 2/3 (i.e., 0.66), causes the distribution to function essentially as a polydisperse distribution and, if smaller than 2/3, causes the distribution to function as an essentially monodisperse distribution.
Accordingly, it is another object of the present invention to provide a polydisperse liquid crystal material including a broad distribution of liquid crystal microdroplets having a spread parameter at least 0.66.
Lastly, it is another object of the present invention to provide a method for conveniently and economically forming a polymer-dispersed liquid crystalline material having a multimodal droplet size distribution with local maxima at two or more droplet diameter sizes.